Hatching in the teleost Oryzias latipes: limited proteolysis causes egg envelope swelling

Homogenates of hatching gland explants were fractionated by means of different electrophoretic techniques, and the fractions analyzed for proteolytic activity and for their capacity to affect the envelope of the eggs. Agar gel electrophoresis resulted in two fractions that were able to digest the zona radiata interna, and a third fraction that caused a significant swelling of the egg envelope. All three fractions were proteolytically active. Agarose gel and polyacrylamide gel electrophoresis gave only two fractions that showed proteolytic activity and were capable of digesting the zona radiata interna. The presence of these two fractions may imply that hatching enzyme is stored as proenzyme in the hatching gland granules. Swelling of egg envelopes was also observed during envelope digestion by thermolysin, pronase, and 0.5-1.0 N NaOH. Moreover, breakdown by hatching enzyme and pepsin under suboptimal conditions showed a slight swelling of the envelope. These results demonstrate that substances capable of solubilizing the zona radiata interna may cause envelope swelling. The swelling of the envelopes probably represents an intermediate phase in the proteolysis of the zona radiata interna. The agar gel electrophoresis fraction of hatching gland homogenates that causes swelling may contain a physicochemically different form of hatching enzyme.